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Lecture 3 #1
Hubs, Bridges and Switches
Lecture 3
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Lecture 3 #2
Interconnecting LANs
Q: Why not just one big LAN? Limited amount of supportable traffic: on single
LAN, all stations must share bandwidth
limited length: 802.3 (Ethernet) specifiesmaximum cable length
large collision domain (can collide with manystations)
limited number of stations: 802.5 (token ring)have token passing delays at each station
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Lecture 3 #3
Hubs Physical Layer devices: essentially repeaters
operating at bit levels: repeat received bits on oneinterface to all other interfaces
Hubs can be arranged in a hierarchy (or multi-tier
design), with backbone hub at its top
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Lecture 3 #4
Hubs (more)
Each connected LAN referred to as LAN segment
Hubs do not isolate collision domains: node may collidewith any node residing at any segment in LAN
Hub Advantages: simple, inexpensive device
Multi-tier provides graceful degradation: portionsof the LAN continue to operate if one hub
malfunctionsextends maximum distance between node pairs
(100m per Hub)
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Lecture 3 #5
Hub limitations
single collision domain results in no increase in maxthroughput
multi-tier throughput same as single segment
throughput individual LAN restrictions pose limits on number
of nodes in same collision domain and on totalallowed geographical coverage
cannot connect different Ethernet types (e.g.,10BaseT and 100baseT) Why?
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Lecture 3 #6
Bridges
Link Layer devices: operate on Ethernetframes, examining frame header andselectively forwarding frame based on its
destination Bridge isolates collision domains since it
buffers frames
When frame is to be forwarded onsegment, bridge uses CSMA/CD to accesssegment and transmit
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Lecture 3 #7
Bridges (more)
Bridge advantages: Isolates collision domains resulting in higher
total max throughput, and does not limit the
number of nodes nor geographical coverage
Can connect different type Ethernet since it isa store and forward device
Transparent: no need for any change to hostsLAN adapters
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Lecture 3 #8
Backbone Bridge
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Lecture 3 #9
Interconnection Without Backbone
Not recommended for two reasons:
- single point of failure at Computer Science hub- all traffic between EE and SE must path over
CS segment
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Lecture 3 #10
Bridges: frame filtering, forwarding
bridges filter packets same-LAN -segment frames not forwarded onto
other LAN segments
forwarding:how to know on which LAN segment to forward
frame?
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Lecture 3 #11
Bridge Filtering
bridges learn which hosts can be reached throughwhich interfaces: maintain filtering tables
when frame received, bridge learns location of
sender: incoming LAN segment records sender location in filtering table
filtering table entry:
(Node LAN Address, Bridge Interface, Time Stamp)
stale entries in Filtering Table dropped (TTL can be60 minutes)
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Lecture 3 #12
Bridge Operation
bridge procedure(in_MAC, in_port,out_MAC)Set filtering table (in_MAC) to in_port /*learning*/lookup in filtering table (out_MAC) receive out_portif
(out_port not valid) /* no entry found for destination */then flood; /* forward on all but the interface onwhich the frame arrived*/
if (in_port = out_port) /*destination is on LAN on whichframe was received */
then drop the frame
Otherwise (out_port is valid) /*entry found for destination */then forward the frame on interface indicate
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Lecture 3 #13
Bridge Learning: example
Suppose C sends frame to D and D replies back withframe to C
C sends frame, bridge has no info about D, sofloods to both LANs bridge notes that C is on port 1
frame ignored on upper LAN
frame received by D
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Lecture 3 #14
Bridge Learning: example
D generates reply to C, sends
bridge sees frame from D
bridge notes that D is on interface 2
bridge knows C on interface 1, so selectivelyforwards frame out via interface 1
C 1
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Lecture 3 #15
What will happen with loops?
Incorrect learning
A
B
1 1
22
A , 1 A , 122
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Lecture 3 #16
What will happen with loops?
Frame looping
A
C
1 1
22
C,?? C,??
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Lecture 3 #17
What will happen with loops?
Frame looping
A
B
1 1
22
B,2 B,1
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Lecture 3 #18
Loop-free: tree
A
B
C
A message from Awill mark As location
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Lecture 3 #19
Loop-free: tree
A
B
C
A message from Awill mark As location
A: q
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Lecture 3 #20
Loop-free: tree
A
B
CA: q
A: q
A message from Awill mark As location
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Lecture 3 #21
Loop-free: tree
A
B
CA: qA:p
A:n
A:n
A: q
A message from Awill mark As location
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Lecture 3 #22
Loop-free: tree
A
B
CA: qA:p
A:n
A:n
A: q
A message from Awill mark As location
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Lecture 3 #23
Loop-free: tree
A
B
C
A: q
A: qA:p
A:n
A:n
So a message toA will go by marks
A message from Awill mark As location
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Lecture 3 #24
Bridges Spanning Tree
for increased reliability, desirable to haveredundant, alternative paths from source to dest
with multiple paths, cycles result - bridges maymultiply and forward frame forever
solution: organize bridges in a spanning tree bydisabling subset ofinterfaces
Disabled
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Lecture 3 #25
Introducing Spanning Tree
Allow a path between every LAN withoutcausing loops (loop-free environment)
Bridges communicate with specialconfiguration messages (BPDUs)
Standardized by IEEE 802.1D
Note: redundant paths are good, active redundant paths are bad(they cause loops)
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Lecture 3 #26
How to construct a spanning tree?
Bridges run a distributed spanning treealgorithmSelect what ports (and bridges) should actively
forward frames Standardized in IEEE 802.1 specification
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Lecture 3 #27
Overview of STP
We make a series of simplifications:
Build a ST of bridges (in fact, need tospan LAN segments!)
Assume that we are given a root bridge
So we solve in order:
1. How to find a root bridge?
2. How to compute a ST of bridges?3. How to compute a ST LAN segments?
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Lecture 3 #28
1. Choosing a root bridge
Assume each bridge has a unique identifier
Each bridge remembers best ID seen sofar (my_root_ID)
Periodically, send my_root_ID to allneighbors (flooding)
When receiving ID, update if necessary
Is that enough?!
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Lecture 3 #29
2. Compute ST Given a root
Idea: each node finds its shortest paths tothe root shortest paths tree
Output: At each node, parent pointer (and
distance)How: Bellman-Ford algorithm
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Lecture 3 #30
Distributed Bellman-Ford
Assumption: There is a unique root node sIdea: Each node, periodically, tells all its
neighbors what is its distance from s
But how can they tell? s: easy. dist
s= 0 always!
Another node v:
Mark neighbor with least distance asparent
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Lecture 3 #31
Why does this work?
Suppose all nodes start with distance g,and suppose that updates are sent everytime unit.
CD
B
E
F
G
A0
gg
g
g
g
g
g
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Lecture 3 #32
Why does this work?
Suppose all nodes start with distance g,and suppose that updates are sent everytime unit.
CD
B
E
F
G
A0
1 1
g
1
1
g
g
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Lecture 3 #33
Why does this work?
Suppose all nodes start with distance g,and suppose that updates are sent everytime unit.
CD
B
E
F
G
A0
1 12
1
1
g
2
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Lecture 3 #34
Why does this work?
Suppose all nodes start with distance g,and suppose that updates are sent everytime unit.
CD
B
E
F
G
A0
1 12
1
1
3
2
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Lecture 3 #35
Bellman-Ford: properties
Works for any non-negative link weightsw(u,v):
Works when the system operatesasynchronously.
Works regardless of the initial distances!(later...)
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Lecture 3 #36
3. ST of LAN segments
Assumption: given a ST of the bridges
Idea: Each segment has at least one bridgeattached. Only one of them should forward
packets! Choose bridge closest to root. Break ties by bridge ID(and then by port ID...)
Implementation: Bridges listen to all distanceannouncement on each port. Mark port as
designated port iff best on that ports LAN
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Lecture 3 #37
Spanning Tree Concepts:P
ath Cost A cost associated with each port on eachbridge (weight of the segment)default is 1
The cost associated with transmission ontothe LAN connected to the portCan be manually or automatically assigned
Can be used to alter the path to the root bridge
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Lecture 3 #38
Spanning Tree Concepts:
RootP
ort Each non-root bridge has a Root port: Theport on the path towards the root bridgeparent pointer
The root port is part of the lowest costpath towards the root bridge
If port costs are equal on a bridge, theport with the lowest ID becomes root port
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Lecture 3 #39
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation:1. Pick a root2. Each bridge picks a
root port
B8
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Lecture 3 #40
Example Spanning Tree
B3
B5
B7B2
B1
B4B6
Root
B4 B5 B6
B8
B1
Spanning Tree:
rootport
B3
B7B2
B8
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Lecture 3 #41
Spanning Tree Concepts:Designated Port Each LAN has a single designated port
This is the port reporting minimum costpath to the root bridge for the LAN
Only designated and root ports remainactive!
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Lecture 3 #42
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Forwarding Tree:
DesignatedBridge
rootport
Note: B3, B6 forward nothing
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Lecture 3 #43
Spanning Tree Requirements
Each bridge has a unique identifier
A broadcast address for bridges on a
LAN A unique port identifier for all ports
on all bridgesBridge id + port number
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Lecture 3 #44
Spanning Tree Algorithm:ImplementationKeep pumping a single message:
(my root ID, my cost to root, my ID)
BPDU: Bridge Protocol Data Unit
Update vars when receiving: My_root_ID: smallest seen so far
My_cost_to_root: smallest received to my_root +link cost
Break ties byID
Thats enough!
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Lecture 3 #45
Spanning Tree Algorithm:Select Designated Bridges
Bridges send BPDU frames to its attachedLANs
sender portIDbridge and port ID of the bridge the sending
bridge considers rootroot path cost for the sending bridge
3. Best bridge wins, and it knows it (andwinning port) (lowest ID/cost/priority)
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Lecture 3 #46
Forwarding/Blocking State
1. Only root and designated ports areactive for data forwarding Other ports are in the blocking state:
no forwarding!
If bridge has no designated port, noforwarding at all block root port too.
2. All ports send BPDU messages To adjust to changes
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Lecture 3 #47
Spanning Tree Protocol: Execution
B3
B5
B7B2
B1
B6 B4
B8
(B1,root=B1, dist=0)(B1,root=B1,dist=0)
(B4, root=B1, dist=1)(B6, Root=B1dist=1)
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Lecture 3 #48
Bridges vs. Routers
both store-and-forward devices routers: network layer devices (examine network layer
headers)
bridges are Link Layer devices
routers maintain routing tables, implement routingalgorithms
bridges maintain filtering tables, implementfiltering, learning and spanning tree algorithms
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Lecture 3 #49
Routers vs. Bridges
Bridges + and -
+ Bridge operation is simpler requiring lessprocessing
- Topologies are restricted with bridges: a spanningtree must be built to avoid cycles
- Bridges do not offer protection from broadcaststorms (endless broadcasting by a host will beforwarded by a bridge)
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Lecture 3 #50
Routers vs. Bridges
Routers + and -
+ arbitrary topologies can be supported, cycling islimited by TTL counters (and good routing protocols)
+ provide firewall protection against broadcast storms- require IP address configuration (not plug and play)
- require higher processing
bridges do well in small (few hundred hosts) whilerouters used in large networks (thousands of hosts)
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Lecture 3 #51
Ethernet Switches
layer 2 (frame) forwarding,filtering using LANaddresses
Switching: A-to-B and A-to-B simultaneously, nocollisions
large number of interfaces
often: individual hosts,star-connected into switchEthernet, but no
collisions!
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Lecture 3 #52
Ethernet Switches
cut-through switching: frame forwardedfrom input to output port without awaitingfor assembly of entire frame
slight reduction in latency combinations of shared/dedicated,
10/100/1000 Mbps interfaces
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Lecture 3 #53
Ethernet Switches (more)
Dedicated
Shared
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Lecture 3 #54
Optional: Wireless LAN and PPP
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Lecture 3 #55
IEEE 802.11 Wireless LAN
wireless LANs: untethered (often mobile) networking IEEE 802.11 standard:
MAC protocol
unlicensed frequency spectrum: 900Mhz, 2.4Ghz
Basic Service Set (BSS)(a.k.a. cell) contains:
wireless hosts
access point (AP): basestation
BSSs combined to formdistribution system (DS)
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Lecture 3 #56
Ad Hoc Networks
Ad hoc network: IEEE 802.11 stations candynamically form network without AP
Applications:
laptop meeting in conference room, carinterconnection of personal devices
battlefield
IETF MANET(Mobile Ad hoc Networks)working group
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Lecture 3 #57
IEEE 802.11 MAC Protocol:CSMA/CA802.11 CSMA: sender- if sense channel idle for
DISF sec.then transmit entire frame(no collision detection)
-if sense channel busythen binary backoff
802.11 CSMA receiver:
if received OKreturn ACK after SIFSWhy?
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Lecture 3 #58
IEEE 802.11 MAC Protocol
802.11 CSMA Protocol:others
NAV: NetworkAllocationVector
802.11 frame hastransmission time field
others (hearing data)defer access for NAVtime units
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Lecture 3 #59
Hidden Terminal effect
hidden terminals: A, C cannot hear each other
obstacles, signal attenuation
collisions at B
goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance
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Lecture 3 #60
Collision Avoidance: RTS-CTSexchange CSMA/CA: explicit
channel reservation
sender: send shortRTS: request to send
receiver: reply withshort CTS: clear tosend
CTS reserves channel for
sender, notifying(possibly hidden) stations
avoid hidden stationcollisions
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Lecture 3 #61
Collision Avoidance: RTS-CTSexchange RTS and CTS short:
collisions less likely, ofshorter duration
end result similar tocollision detection
IEEE 802.11 allows:
CSMA
CSMA/CA: reservations
polling from AP
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Lecture 3 #62
Point to Point Data Link Control
one sender, one receiver, one link: easierthan broadcast link:
no Media Access Control
no need for explicit MAC addressinge.g., dialup link, ISDN line
popular point-to-point DLC protocols:
PPP
(point-to-point protocol)HDLC: High level data link control (Data
link used to be considered high layer inprotocol stack!)
PPP D F
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Lecture 3 #63
PPP Design Requirements [RFC1557] packet framing: encapsulation of network-layer
datagram in data link frame
carry network layer data of any network layerprotocol (not just IP) at same time
ability to demultiplex upwards
bit transparency: must carry any bit pattern in thedata field
error detection (no correction)
connection livenes: detect, signal link failure tonetwork layer
network layer address negotiation: endpoint canlearn/configure each others network address
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Lecture 3 #64
PPP non-requirements
no error correction/recovery
no flow control
out of order delivery OK
no need to support multipoint links (e.g.,polling)
Error recovery, flow control, data re-orderingall relegated to higher layers!!!
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Lecture 3 #65
PPP Data Frame
Flag: delimiter (framing)
Address: does nothing (only one option)
Control: does nothing; in the future possible
multiple control fields Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)
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Lecture 3 #66
PPP Data Frame
info: upper layer data being carried
check: cyclic redundancy check (CRC) forerror detection
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Lecture 3 #67
Byte Stuffing
data transparency requirement: data field mustbe allowed to include flag pattern Q: is received data or flag?
Sender: adds (stuffs) extra < 01111101> bytebefore each < 01111110> or data byte
Receiver:Receive 01111101
discard the byte, Next byte is data
Receive 01111110: flag byte
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Lecture 3 #68
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte intransmitted data
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Lecture 3 #69
PPP Data Control Protocol
Before exchanging network-layer data, data link peersmust
configure PPP link (max.frame length,authentication)
learn/configure network
layer information
for IP: carry IP ControlProtocol (IPCP) msgs(protocol field: 8021) toconfigure/learn IPaddress
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Lecture 3 #70
Data Link: Summary
principles behind data link layerservices:error detection, correction
sharing a broadcast channel: multiple access link layer addressing, ARP
various link layer technologiesEthernet
hubs, bridges, switches
IEEE 802.11 LANs
PPP
Chapter 5 Kurose and Ross
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Configuration Messages: BPDU
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